In positron emission tomography (PET) of a radiation tomography device, a sensitivity difference between detectors needs to be calibrated to obtain homogeneous sensitivity of all detectors at the time of obtaining a reconstructed image. Hereinafter, this calibration will be referred to as “sensitivity correction”. A coefficient for sensitivity correction (hereinafter referred to as “sensitivity coefficient”) is calculated from actual data collected using a calibration radiation source, and a method of calculating a sensitivity coefficient has two types.
The first method is a direct method of obtaining sensitivity coefficients for all detector pairs. In the direct method, the sensitivity coefficients of all detector pairs are directly obtained. However, when the number of detectors is large, the number of detector pairs becomes enormous by the square of the number of detectors. Therefore, statistical accuracy per pair is lowered, and the number of sensitivity coefficients becomes enormous.
The next one is an “element-by-element sensitivity correction method” of decomposing each factor that varies sensitivity and setting a product thereof to a sensitivity coefficient of a detector pair (for example, see Patent Documents 1 and 2 and Non-Patent Documents 1 to 3). In this element-by-element sensitivity correction method, when a pair of coinciding detector rings is set to (u, v), and a pair of coinciding detectors in a ring is set to (i, j), a sensitivity correction coefficient NCuivj is decomposed into elements as in the following Equation (1).NCuivj=εui×εvj×buvk×duvrk×guvr×fuv  (1)
Here, In Equation (1), εui and εvj denote sensitivities unique to detectors, duvrk denotes a crystal interference factor, fuv denotes ring pair sensitivity, buvk denotes a block profile factor, guvr denotes a radial direction geometric factor, k denotes a crystal relative position in a block, and r denotes a radial direction position. In these elements, guvr, duvrk, etc. are geometrically determined elements (hereinafter referred to as “geometric factors”). In addition, εui, εvj, and buvk are non-geometric factors which change over time.
In the element-by-element sensitivity correction method, the product of sensitivity coefficients is used for expression, and thus the number of sensitivity coefficients is small when compared to the direct method. In addition, since each factor obtained by decomposing the sensitivity coefficient is added using geometric symmetry, statistical accuracy can be improved. However, the sensitivity coefficient is indirectly obtained by the product of factors, and thus is an approximate numerical value. In a recent PET apparatus, the element-by-element sensitivity correction method starts to be used for the purpose of high resolution, simplification of calibration and improvement of statistical accuracy.
A calibration flow of a conventional element-by-element correction method is illustrated in FIG. 1 and FIG. 2 of Non-Patent Document 1. FIG. 1 is a calibration flow for calculation of a geometric factor using a conventional low scattering calibration radiation source and FIG. 2 is a calibration flow for calculation of a non-geometric factor using a conventional uniform cylindrical calibration radiation source. The geometric factor which does not change over time is calculated using the low scattering calibration radiation source, and the non-geometric factor which changes over time is calculated using data of the uniform cylindrical calibration radiation source corrected by a coefficient of the geometric factor. Then, a sensitivity coefficient of a specific pair of detectors is obtained using Equation (1) shown above. With regard to a specific calibration flow, refer to Non-Patent Document 1.